This report describes a study of the immobilization of Cs+, Cd2+ Pb2+ and Cr3+ by synthetic zeolites formed as a result of the hydrothermal treatment of Spanish class F coal fly ash in a 1 M solution of NaOH. The majority zeolite formed at 150 degrees C was a gismondine-type P1-Na species (Na(6)Al(6)Si(10)O(32)center dot 12H(2)O), which at 200 degrees C transformed into analcime-C zeolite (Na(Si2Al)O6H2O). The shift in pore size distribution towards pores with diameters of about 2.2 nm observed after the zeolites were washed entailed an increase in the specific surface area, to values nearly double the figures recorded prior to washing. With a high selectivity for Cs, the gismondine type Na zeolite P was found to be the best candidate for immobilizing radioactive waste. Gismondine and analcime-C zeolites also exhibited high Cd selectivity. (C) 2005 Elsevier Ltd. All rights reserved.